[forwarded from http://bugs.debian.org/268115]
Matthias
The bug submitter writes:
compiling this function:
double baz(double foo, double bar)
{
return foo*foo*foo*foo*bar*bar*bar*bar;
}
on amd64 with -O6 -ffast-math, gcc emits this code:
foo.o: file format elf64-x86-64
Disassembly of section .text:
... (some similar functions that I was messing around with) ...
0000000000000050 <ddbar>:
50: f2 0f 59 c0 mulsd %xmm0,%xmm0
54: f2 0f 59 c0 mulsd %xmm0,%xmm0
58: f2 0f 59 c1 mulsd %xmm1,%xmm0
5c: f2 0f 59 c1 mulsd %xmm1,%xmm0
60: f2 0f 59 c1 mulsd %xmm1,%xmm0
64: f2 0f 59 c1 mulsd %xmm1,%xmm0
68: c3 retq
So, it notices that it can do foo*foo*foo*foo with two mulsd instructions,
but it misses the same optimization for bar*bar*bar*bar. It would save one
FP multiply overall to do:
mulsd %xmm0, %xmm0
mulsd %xmm1, %xmm1
mulsd %xmm0, %xmm0
mulsd %xmm1, %xmm1
mulsd %xmm1, %xmm0
retq
Also, the two non-dependent muls could run in parallel.
Without -ffast-math, of course, gcc can't take advantage of the laws of
arithmetic like that and has to do all the multiplies the straightforward
way.
--
Summary: could optimize FP multiplies better
Product: gcc
Version: 3.4.2
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: target
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: debian-gcc at lists dot debian dot org
CC: gcc-bugs at gcc dot gnu dot org
GCC build triplet: amd64-linux
GCC host triplet: amd64-linux
GCC target triplet: amd64-linux
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18589
